It is known to provide solid state light sources in packages that may provide protection, color selection, focusing and the like for light emitted by the light emitting device. For example, the solid state light source may be a light emitting diode (“LED”). Various problems may be encountered during packaging of a power LED for use as a light source. Examples of such possible problems will be described with reference to the cross-sectional illustration of an LED in FIG. 1. As shown in FIG. 1, an LED package 100 generally includes a substrate member 102 on which a light emitting device 103 is mounted. The light emitting device 103 may, for example, be mounted on the substrate member 102 using a submount 101. The LED device may be bonded with wire(s) to connect its terminals to the electrical terminals in the substrate to be powered up. The substrate member 102 may include traces or metal leads for connecting the package 100 to external circuitry. The substrate 102 may also act as a heatsink to conduct heat away from the LED 103 during operation.
A reflector, such as the reflector cup 104, may be mounted on the substrate 102 and surround the light emitting device 103 which may be assembled on the substrate 102 for ease of manufacturability. The reflector cup 104 illustrated in FIG. 1 includes an angled or sloped lower sidewall 106 for reflecting light generated by the LED 103 upwardly and away from the LED package 100. The illustrated reflector cup 104 also includes upwardly-extending walls 105 that may act as a channel for holding a lens 120 in the LED package 100, and a horizontal shoulder portion 108 for directly or indirectly positioning the lens 120 at a desired height above the light emitting device 103. The upwardly extending walls 105 may also help protect the lens 120 from mechanical shock and stress.
As illustrated in FIG. 1, after the reflector 104 is mounted on the substrate 102, an encapsulant material 112, such as liquid silicone gel, is dispensed into an interior reflective cavity 115 defined by the reflector cup 104. The interior reflective cavity 115 illustrated in FIG. 1 has a bottom surface defined by the substrate 102 to provide a closed cavity capable of retaining a liquid encapsulant material 112 therein.
After placement of the lens 120, the package 100 is typically heat-cured, which causes the encapsulant material 112 to solidify and adhere to the lens 120. The lens 120 may, thus, be held in place by the cured encapsulant material 112. However, encapsulant materials having a slight shrinkage factor with curing, such as a silicone gel, generally tend to contract during the heat curing process. In addition, the coefficient of thermal expansion (CTE) effect generally causes higher floating of the lens at elevated temperatures. During cool-down, parts may have a tendency to contract. As the illustrated volume of encapsulant beneath the lens 120 shown in FIG. 2 is relatively large, this contraction may cause the encapsulant material 112 to delaminate (pull away) from portions of the package 100, including the light emitting device 103, a surface of the substrate 102, the upwardly-extending walls 105 of the reflector cup 104 and/or the lens 120 during the curing process. This delamination may significantly affect the optical performance of the package 100, particularly when the delamination is from the die, where it may cause total internal reflection to occur that may trap light in the package 100. This contraction may also create gaps or voids between the encapsulant material 112 and the light emitting device 103, lens 120, and/or reflector cup 104. Tri-axial stresses in the encapsulant material 112 may also cause cohesive tears in the encapsulant material 112. These gaps and/or tears may substantially reduce the amount of light emitted by the light emitting device package 100. The contraction may also pull out air pockets from crevices (e.g., beneath the reflector) or from under devices (e.g., the die/submount), which may then interfere with the optical performance of the package 100.
During operation of the package 100, large amounts of heat may be generated by the light emitting device 103. Much of the heat may be dissipated by the substrate 102 and the reflector cup 104, each of which may act as a heatsink for the package 100. However, the temperature of the package 100 may still increase significantly during operation. Encapsulant materials 112, such as silicone gels, typically have high coefficients of thermal expansion. As a result, when the package 100 heats up, the encapsulant material 112 may expand. As the lens 120 is mounted within a channel defined by the upwardly-extending walls 105 of the reflector cup 104, the lens 120 may travel up and down within the upwardly-extending walls 105 as the encapsulant material 112 expands and contracts. Expansion of the encapsulant material 112 may extrude the encapsulant into spaces or out of the cavity such that, when cooled, it may not readily move back into the cavity. This could cause delamination, voids, higher triaxial stresses and/or the like, which may result in less robust light emitting devices. Such lens movement is further described, for example, in United States Patent Application Pub. No. 2004/0041222.
In addition, during operation of the device, some light is emitted by the light emitting device 103 toward the lower sidewalls 106 of the reflector cup 104. Most light incident upon the lower sidewalls 106 will be reflected upward and out of the optical cavity 115 defined by the reflector cup 104. However, some of the light incident upon the lower sidewalls 106 will be absorbed, leading to increased heating and/or optical losses. For example, in typical LED packages, the reflector cup may include a metal such as copper plated with a reflective metal such as silver, or an injection molded plastic coated with a reflective metal layer, such as an aluminum or silver layer. A highly specular silver surface may reflect only about 96% of incident light, while about 4% of the incident light may be absorbed. Furthermore, as the metal surface oxidizes over time, the reflectivity of the metal may decrease further.
The presence of the relatively high vertical walls 105 for mechanical protection and lens alignment may further contribute to optical losses in the package 100 and/or may result in an undesirable light emission pattern.